The invention concerns a material which, when applied in a thin coating to a transparent support, has phototropic properties.
Phototropic glasses and their properties have been extensively described in numerous patents. There is a recent summary of the state of the art: Gliemeroth und Mader: Angew. Chemie 9 (1970) 6, 434, International Edition in English. Such glasses contain the agents of phototropy uniformly distributed in the form of mostly glassy segregations (Bach und Gliemeroth: Glastechn. Ber. 44 (1971) 8, 305, or J. Amer. Ceram. Soc. 54 (1971) 10).
In contrast to homogeneous, phototropic, inorganic glasses of this kind, phototropic, thin inorganic coatings which can be applied in simple form to a transparent support (hereinafter called the substrate) are unknown, so far as is known to the applicant. So far as applicant is informed, there is no suitable phototropic coating material.
The most obvious courses--the use of a coaating of silver halogen--does not produce reversible phototropy: in the most favorable case it produces naught but photosensitivity, such as is known in the photographic art. The search for suitable inorganic materials for phototropic coatings has thus far led to only two practical possibilities so far as known to applicant.
From German Offenlegungsschrift 1,596,819 and British Patent 1,111,740, work has become known in which a coating of photosensitive silver halide was applied to a substrate of glass or plastic and then covered with a transparent acceptor coating. This acceptor coating serves for the capture of the halogens released upon illumination and assures their return to the photosensitive silver halide layer, thus providing for the reversibility of the phototropic process. Without the acceptor layer, the loss of the halogens by diffusion would result in a fatiguing of the phototropy (British Pat. 1,111,749, p. 2, 105 et seq.). The essential content of this work according to German Offenlegungsschrift 1,596,819 and British Pat. 1,111,740 is the combination of a coating of silver halide (wholly or partially replaceable by likewise photosensitive mercury halide or thallium halide) with an acceptor substance which in the form of a coating must cover the photosensitive layer. Experience has shown that such combination entails difficulties in manufacture, so that a simpler solution must be sought.
Another approach to the production of phototropic coatings has been taken in German auslegeshrift 1,596,764 (U.S. Pat. No. 3,419,370) by doping a halogen-containing, silver-free glass in a layer with silver, so that in this layer silver halides were formed which produced the phototropic effect. Here the blocking of the halogens which is necessary to prevent fatiguing of the reversibility, by preventing loss through diffusion, is accomplished by the surrounding glass matrix (just as in solid phototropic glasses). Silver doping is performed preferably by ion exchange (against sodium ions in the base glass). This process, however, has the disadvantage of involving a doping process and a heat treatment which cannot be performed in the framework of common coating processes.
A variety of materials have already been proposed for electrochromatic layers, e.g., in U.S. Pat. No. 3,521,941 and U.S. Pat. No. 3,512,869. In both cases photosensitive materials are sandwiched between transparent electrode layers. These known layers are especially characterized by the fact that they have an additional acceptor layer for any decomposition products that may develop upon exposure of the electrochromatic layer to light.